In about 1973 linear field gradients were first used with NMR equipment to "locate" the resonant frequency response. This expedient made it possible to reconstruct images from free induction decay (FID) responses occasioned by temporarily subjecting objects located in strong static magnetic fields to rotating radio frequency fields. Since then scientists in the field have been attempting to find coil arrangements that provide the most stable, homogeneous, orthogonal and linear gradient fields.
Among other things the gradient fields have to be switched on and off in short time periods, thus it is desirable that they have relatively low inductances.
The prior art gradient generating coils comprise, among other things, four theoretically infinite conductors equidistantly spaced apart in each of the four quadrants of the plane defined by the direction of the static field and the directions of the gradient. The conductors are normal to that plane. Thus, a gradient .gradient..sub.x is generated by conductors normal to the XZ plane. Practically of course the gradient coil conductors are finite and the coils have been described aptly by their finite shapes as "trapezoidal", "rectangular", "saddle" and the like.
To keep the inductance low the prior art gradient coils have had a relatively small number of turns. The small number of turns, of course acts to reduce the gradient magnetic field strength. The use of the coils unfortunately have have a deleterious effect on the decay which in turn adversely affects the detected FID signals. Also, the coils inherently have resistance and therefore generate heat which is wasted energy. In addition to resistive generated heat there are eddy current losses in the coils that increase the power consumption. Thus the prior art gradient generating coils adversely affect the FID signals while locating them. The gradient coils also introduces power losses.
Both the linearity and orthonogality of the field generated by a coil are a function of the geometry of the system, being contingent on such details as the geometrical location of each of the turns relative to other turns, among other things. It is difficult to obtain geometrical relationships, with the prior art gradient coils, that ensure linearity and orthonogality. Accordingly there is a real present need for improved gradient field generating arrangements.